GEOLOGY

New Geology articles posted online ahead of print 19 March 2012

Boulder, CO, USA – Highlights include evidence of the sensitivity of Greenland Ice Sheet outlet glaciers to rising air temperatures and a call for further understanding of the bathymetry beneath them before accurate predictions of sea-level rise can be made; findings in the North Sea of the largest body of sand on Earth, large enough to cover the whole of London six meters deep; and the unexpected discovery of the oldest evidence of animals burrowing in search of food.

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The Ediacaran Period, an interval in Earth's history after the Snowball Earth glaciations but before the Cambrian radiations, marks the introduction of complex macroscopic organisms synchronously in unrelated groups. It has been proposed that the increase in size in marine organisms was triggered by the oxygenation of Ediacaran oceans. New research shows that animals, rather than a late Neoproterozoic increase in oxygen concentration, could equally lead to a revolution in the structure and evolution of marine paleocommunities. In the course of studying Ediacaran-age rocks in a remote region of arctic Siberia, Vladimir Rogov and fellow researchers from Institute of Petroleum Geology and Geophysics in Novosibirsk (Russia) unexpectedly came across the oldest evidence of bioturbation (disruption of fine-laminated sediments by purposeful burrowing of animals in search for food) that significantly precedes the Cambrian radiations. Of special interest is that the advent of bioturbation in the fossil record coincides with the earliest ecological differentiation of macroscopic Ediacaran communities, which is interpreted to be a direct consequence of ecosystem engineering by animals. Bioturbation in modern seafloor habitats substantially affects key ecosystem process, including biogeochemical interactions, nutrient cycling, and primary productivity; the first appearance and expansion of bioturbation in the Ediacaran, therefore, must have had a profound effect on ecosystem structure and functioning.

Yaron Be'eri-Shlevin of the Swedish Museum of Natural History and colleagues present findings of an approximately one billion year old (Ga) island arc at the northern part the Arabian-Nubian Shield (ANS). Volcanic-platonic rocks from the Sa'al metamorphic complex (SMC) in Sinai, Egypt, have island arc geochemical affinity and yield zircon U-Pb ages of ~1.02-1.03 Ga. They contain zircon xenocrysts of dominantly 1.11 Ga and a few of Paleoproterozoic age, suggesting the existence of an older arc predating the Sa'al arc by about 80 million years. Detrital zircons of the SMC pelites exhibit textural and U-Pb age patterns supporting their derivation from the volcanic rocks as arc detritus. The ~820 million year age of a granitic pluton intruding the complex suggests that by Cryogenian times the 1 Ga rocks were already part of the ANS. The 1.0-1.1 Ga SMC rocks provide a possible connection between latest Mesoproterozoic ocean closure during the assembly of Rodinia and the later buildup of Gondwana. There is growing indication, including the findings of this study, that 1.0–1.1 Ga crust composed a more significant component in northernmost Gondwana than hitherto recognized.

Kenneth G. Miller of Rutgers University and colleagues demonstrate that the available data constrain peak sea level at 22 meters (plus or minus 10 m -- a 95% confidence interval) during a warm interval about 3 million years ago, when atmospheric carbon dioxide levels were similar to today's and temperatures were 2-3 degrees Celsius warmer. This indicates that the equilibrium condition for sea level under today's atmospheric CO2 levels requires the nearly total deglaciation of both Greenland and the West Antarctic Ice Sheet, with a possible contribution of ~10 m from the low-lying, marine-based coastal margins of the East Antarctic Ice Sheet.

Volume determination of tephra deposits is necessary to the characterization of active volcanoes, with obvious implications for environmental and climatic impact, estimation of magma-production rate, long-term hazard assessments, and forecasting of future eruptions. Several methods have been proposed that mainly include the integrations of various deposit-thinning relationships and the inversion of field observations using computational models. Regardless of their strong dependence on tephra-deposit exposure, empirical integrations of deposit-thinning trends still represent the most widely adopted strategy due to their practical and fast application. The choice of best-fitting trends (e.g., exponential and power-law thinning on semi-log plots) has been the subject of lively debate because they are all characterized by various advantages and disadvantages. C. Bonadonna of the University of Geneva and A. Costa of the University of Reading propose a new empirical method based on the Weibull distribution, which shows a better agreement with observed data, reconciling the debate on the use of the exponential versus power-law methods. Nonetheless, Bonadonna and Costa also show that all empirical methods used to derive erupted volume based on integration of deposit thinning strongly depend on the available data and are affected by various degrees of uncertainties. Application of various empirical and analytical methods can help to assess the associated uncertainties.

The most common volcanoes on the surface of Earth, small-volume, short-lived basaltic volcanoes, offer substantial opportunities for learning how magma behaves in the critical transition from coherent magma in dikes to pyroclastic dispersions. Spatter-dikes at the Castle Butte Trading Post volcanic complex (Hopi Buttes volcanic field, Arizona, USA), exposed about 150 m below the paleosurface, present an outstanding record of this transition. They record pulsatory magma rise and eruption, fissure extension, vent stepping, and conduit wall-rock failure, providing a critical link to activity during historic eruptions of well-observed and well-monitored large-volume basaltic volcanoes. N.S. Lefebvre of the University of Otago, New Zealand, and colleagues offer a detailed study of the Castle Butte Trading Post volcanic complex.

Recent changes at the margins of the Greenland Ice Sheet have raised concerns about its future stability -- specifically, the potential for a sudden increase in the ice sheet's contribution to rising global sea levels. The past behavior of the ice sheet allows Anna L.C. Hughes of Swansea University (GLIMPSE Project) and colleagues to investigate what may be driving these changes and how rapidly they can occur. The team shows that at the end of the last glaciation, Helheim Glacier retreated ~80 km within 1000 years following abrupt rises in air temperatures across Greenland. The slowest rate these results represent is similar to the fastest observed retreat rates of modern-day Greenlandic glaciers. Measurement limitations mean that Hughes and colleagues cannot distinguish whether retreat rates were potentially much faster than present or sustained for the entire millennium. In either case, results represent rapid and sustained changes to the ice sheet and a period of significant sea-level contribution. Hughes and colleagues suggest that this phase of glacial retreat progressed without pause due to the depth of the fjord underlying the glacier. Their results highlight the sensitivity of marine-terminating glaciers to rising air temperatures as well as the need to know the bathymetry beneath the outlet glaciers of the Greenland Ice Sheet before robust predictions of sea-level rise can be made.

New data indicate that "waves" in the sediments in the Sea of Marmara formed by slow-motion landsliding, wherein sediments slowly creep downhill over hundreds of thousands of years, creating wrinkles in the sedimentary layers. The Sea of Marmara straddles the North Anatolian Fault, an active strike-slip fault in northern Turkey that poses a serious seismic hazard to Istanbul. Researchers D.J. Shillington of the Lamont-Doherty Earth Observatory and colleagues note that it is important to understand the sedimentary features here because they host a record of past motions on the North Anatolian fault and other faults in the area. In this case, they report, movement on the faults shapes the topography of the seafloor, creating the slopes necessary for the sediments to creep downhill. As a result, the wrinkles formed by creep might be a clue to past fault activity. The gradual downhill movement of sediments by creeping may also reduce the potential for catastrophic landslides triggered by earthquakes.

Tim M. O'Brien and Ben A. van der Pluijm of the University of Michigan present 40Ar/39Ar step-heating analyses for pseudotachylytes from a Neoproterozoic normal fault system belonging to the St Lawrence Rift. Their analysis shows that these "fault rocks" preserve the timing of rifting and initial opening of the Iapetus Ocean. Ten analyses from two pseudotachylytes with varying matrix/clast ratios yield ages of 634.7 (plus or minus 1.6) to 663.9 (plus or minus 1.8) million years old. These ages show a linear relationship with the proportion of clast inclusions, resulting in lower intercept ages (i.e., no host rock) of 613.3 (plus or minus 5.1) and 614.2 (plus or minus 3.0) million years. These ages constrain major seismic faulting along the St Lawrence Rift System and significantly improve prior estimates for late Neoproterozoic rifting of Iapetus. The upper intercepts, reflecting host rock ages, match cooling ages of Grenville basement in the area. O'Brien and van der Pluijm conclude that the time of major continental rifting along the northern Laurentian margin and initiation of the Iapetus Ocean occurred circa 613–614 million years ago.

Aeolian ripples, which form regular patterns on sand beaches and desert floors, indicate the fundamental instability of flat sand surfaces under the wind-induced transport of sand grains. Two kinds of sand ripples exist: normal, small ripples and megaripples with wavelengths reaching up to several meters. They differ also in their grain-size distributions (unimodal for sand ripples and bimodal for megaripples). While sand ripples form almost straight lines, megaripples have greater sinuosity due to their transverse instability, a property that causes small megaripple undulations to grow with time. The origin of the instability is due to variations in megaripple height, which do not diminish over time, as well as to the inverse dependence of ripple drift velocity on height. Thus, the taller regions of ripples will move more slowly than the adjacent, shorter portions, an outcome that promotes further perturbation growth. Hezi Yizhaq of Ben-Gurion University of the Negev and colleagues provide an example, based on field work, of the transverse instability of megaripples. The instability growth rate depends on the difference between the heights of the different segments of the megaripple. Their results suggest a physical mechanism for the transverse instability of megaripples and new insight into the spatial patterns of sand ripples.

Carbonate ultracataclasite (CUC) is found as a veneer along the bedding plane portion of the Eocene Heart Mountain detachment (Wyoming, United States). At White Mountain, where the CUC is thickest, John P. Craddock of Macalester College (Minnesota) and colleagues report the discovery of six dikes, as much as 1 m wide, that were intruded vertically ~120 m from the detachment into the overlying Madison and Bighorn Formations.

Using 3-D seismic and well data from the northern North Sea, Helge Løseth of the Statoil Research Center (Trondheim, Norway) and colleagues describe a large (10 cubic kilometers) body of sand and interpret it as extrusive. The authors note that to their knowledge, this is the world's largest such sand body, large enough to bury Manhattan (60 square kilometers) under 160 m of sand, or the whole of London (1579 square kilometers) under 6 m of sand. This represents a new type of economically interesting reservoir. The sand vented to the seafloor when it was more than 500 m deep. Currently, the sand (1) covers an area of more than 260 square kilometers; (2) is up to 125 m thick; (3) wedges out, away from a central thick zone; and (4) is locally absent along irregular ditches, 20 km long and up to 50 m deep, which overlie feeders on the flanks of the mounds. High fluid pressure fractured the regional seal in the study area so that fluidized sand moved rapidly to the seafloor through fissures, mixed with seawater, and formed lateral gravity currents. These transported the sand up to 8 km away from the blow-out fissures and formed extruded sand sheets.